1. Field of the Invention
This invention relates to a sensor arrangement comprising an optical waveguide having a core of material which is transmissive for optical radiation having a given wavelength and a cladding which surrounds the core, the cladding being of material which has a refractive index at the given wavelength which is less than the refractive index of the adjacent surface of the core, the arrangement including a sensor material which is capable of taking up a given substance or substances within itself and has a refractive index which is dependent upon the amount of such substance(s) taken up, the sensor material being positioned sufficiently close to the core as to be capable, at least when the refractive index of the sensor material has a first value corresponding to a first said amount of substance(s) taken up, of coupling radiation having the given wavelength out from the core and thereby reducing the transmissivity of the waveguide for radiation having the given wavelength.
2. Related Art
An arrangement of this general kind is known, for example, from U.S. Pat. No. 4,634,856. The known arrangement is for sensing moisture and comprises an optical fibre core surrounded by cladding material. The cladding material has a refractive index which is a function of its moisture content and may be constituted by sintered or porous plastic. In this known arrangement the sensor material and the cladding material are therefore one and the same. It appears that the refractive index of the dry plastic is less than that of water and that the effective refractive index increases towards that of the core with increasing moisture content. Light signals are transmitted through the fibre from a light source at one end to a detector unit at the other. The increase in the refractive index of the cladding towards that of the core occurring with increasing moisture content results in a modification of the modal power distribution in the core and hence in attenuation of the light signals received by the detector unit. It appears that, for this to occur, the fibre operates as a multimode waveguide. The specification mentions that information on optical signal loss throughout the length of the fibre can be obtained by means of an optical time-domain reflectometer (OTDR). Thus moisture content can be measured at any point along the entire length of an elongated optical fibre arranged about or along a substantial area being monitored. Moisture measurements can be made at many points along a single optical fibre cable containing numerous individual optical fibres for monitoring at the desired locations.
Single-mode optical fibres are being increasingly employed to carry signals in telecommunications networks. Multi-fibre cables are conventionally used, couplings being made to individual fibres at convenient "splitter" nodes. Because the operation of these nodes tends to be degraded by moisture, steps have to be taken to minimise this risk, for example by hermetically sealing the nodes and packing them with silica gel. Even so there is still a finite risk of the relative humidity within the nodes rising to an unacceptable level. If this should occur it is highly desirable that the situation be detected promptly, so that remedial action can be taken before unacceptable degradation occurs.
It is of course possible to provide one of the many known humidity sensors in each node and interrogate it, and power it also if necessary, from a central monitoring point by means of electrical wiring. However this is cumbersome and costly and there is a need for a humidity sensor which can be included in, and monitored via, a length of single-mode optical fibre. Preferably too it should be possible to construct such a sensor in such a way that it does not result in an unacceptably large attenuation of signals passing through the fibre, whatever the ambient humidity is, so that several such sensors can be included at successive locations along the same fibre and be satisfactorily monitored via that fibre.